Patent GB-966384-A (GB)
Title: Improvements in or relating to amplitude compression arrangements suitable for use inelectrical signalling systems
Inventor: DAVIS JOHN CHRISTOPHER HAMMOND
IPC: H03M1/00
Filed: 1960-03-22
Abstract: 966,384. Pulse code modulation systems; transistor circuits. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. June 22, 1961 [March 22, 1960], No. 10142/60. Headings H3T and H4L. In a pulse code modulation system coding is effected by a plurality of series-connected twostate current or voltage dividing circuits or attenuators which are controlled in accordance with successive outputs from a comparator to which is supplied the incoming signal and a comparison current or voltage dependent on the condition of the dividing circuits. The system is arranged to provide amplitude compression according to a predetermined law. The encoder includes toggles T01 to T07, T01 corresponding to the sign digit and T02 to T07 to the six digits of the binary code. Timing pulse T8 sets toggle TO and resets the others which are set in succession by pulses T2 to T7. Incoming signals IP are supplied via a distributer (not shown) to the comparator C and if an output is obtained from the comparator, no output is obtained from an inverter I and toggle T01 remains set during the coding sequence. If no output is obtained from the comparator, the inverter I provides an output to reset toggle T01 via gates G1, G2, G4 during pulse T2 and the toggle T01 remains reset for the rest of the coding sequence. Toggles T02 to T07 control circuits D2 to D6 which are connected in series and in which the current may be divided so that only a predetermined portion passes to the next circuit, current of predetermined value being applied to the divide circuit D7 and circuit D2 being connected via a reversing switch RS, controlled by the sign toggle T01, to the comparator C. To ensure that the dividing chain D2 to D7 gives a zero result in its minimum position the remaining residual current (as shown 40 ÁA) is extracted by a current switch CS, the operation of the current switch being modified in accordance with the polarity of the signal, Fig. 4 (not shown). The coded output signal may be obtained directly from the comparator C via line LN or may be obtained from the toggles in parallel form. Fig. 2 shows one arrangement for a current dividing circuit D2 &c. When both transistors TR1, TR2 are conducting, current I1 will divide between resistors R1, R2 in accordance with their resistance values. The current from the collector of TR2 forms part of the current 12 which is the input to the next stage, but the collector of TR1 is connected to the emitters of transistors TR3, TR4 which are connected so that only one or other will conduct. Transistor TR4 has a fixed bias applied to its base and when it conducts its current is rejoined with that through TR2 to constitute 12. The base of transistor TR3 is supplied with a potential controlled by the operation of the corresponding toggle and when TR3 conducts the resulting current is diverted from the output. Thus dependent on the operation of the comparator, two possible values of 12 are obtainable. In a modification, Fig. 6 (not shown), the transistor TR1 is dispensed with. In an alternative arrangement, Fig. 3, providing voltage division, the input voltage V1 is supplied to the base of NPN transistor TR1 and the control voltage from the encoder is supplied to the base of NPN transistor TR2, TR2 being arranged when conducting to switch in an additional shunt resistor R2 in the emitter circuit of TR1. Thus, two possible output potentials may be applied to the base of PNP transistor TR3 co-operating in a similar manner with PNP transistor TR4 controlled by the encoder. In a modified encoder, Fig. 7 (not shown), the reversing switch RS operates on the incoming signal instead of the comparison current from the dividing circuits. Specifications 957,503 and 963,831 are referred to.